Gallium nitride bulk crystals and their growth method

ABSTRACT

A gallium nitride crystal with a polyhedron shape having exposed {10-10} m-planes and an exposed (000-1) N-polar c-plane, wherein a surface area of the exposed (000-1) N-polar c-plane is more than 10 mm 2  and a total surface area of the exposed {10-10} m-planes is larger than half of the surface area of (000-1) N-polar c-plane. The GaN bulk crystals were grown by an ammonothermal method with a higher temperature and temperature difference than is used conventionally, using a high-pressure vessel with an upper region and a lower region. The temperature of the lower region is at or above 550° C., the temperature of the upper region is set at or above 500° C., and the temperature difference between the lower and upper regions is maintained at or above 30° C. GaN seed crystals having a longest dimension along the c-axis and exposed large area m-planes are used.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. Section 120 ofcommonly-assigned U.S. Utility patent application Ser. No. 12/234,244,filed on Sep. 19, 2008 Now U.S. Pat. No. 8,253,221, by Tadao Hashimotoand Shuji Nakamura, entitled “GALLIUM NITRIDE BULK CRYSTALS AND THEIRGROWTH METHOD,”, which application claims the benefit under 35 U.S.C.Section 119(e) of commonly-assigned U.S. Provisional Patent ApplicationSer. No. 60/973,662, filed on Sep. 19, 2007, by Tadao Hashimoto andShuji Nakamura, entitled “GALLIUM NITRIDE BULK CRYSTALS AND THEIR GROWTHMETHOD,”, both of which applications are incorporated by referenceherein.

This application is related to the following commonly-assigned U.S.patent applications:

U.S. Utility patent application Ser. No. 11/921,396, filed on Nov. 30,2007, by Kenji Fujito, Tadao Hashimoto and Shuji Nakamura, entitled“METHOD FOR GROWING GROUP-III NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIAUSING AN AUTOCLAVE,”, which application claims the benefit under 35U.S.C. Section 365(c) of PCT Utility patent Application Serial No.US2005/024239, filed on Jul. 8, 2005, by Kenji Fujito, Tadao Hashimotoand Shuji Nakamura, entitled “METHOD FOR GROWING GROUP III-NITRIDECRYSTALS IN SUPERCRITICAL AMMONIA USING AN AUTOCLAVE,”;

U.S. Utility patent application Ser. No. 11/784,339, filed on Apr. 6,2007, by Tadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled“METHOD FOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS INSUPERCRITICAL AMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,”,which application claims the benefit under 35 U.S.C. Section 119(e) ofU.S. Provisional Patent Application Ser. No. 60/790,310, filed on Apr.7, 2006, by Tadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled“A METHOD FOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS INSUPERCRITICAL AMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,”;

U.S. Utility patent application Ser. No. 11/765,629, filed on Jun. 20,2007, by Tadao Hashimoto, Hitoshi Sato and Shuji Nakamura, entitled“OPTO-ELECTRONIC AND ELECTRONIC DEVICES USING N-FACE OR M-PLANE GaNSUBSTRATE PREPARED WITH AMMONOTHERMAL GROWTH,”, now U.S. Pat. No.7,755,172, issued Jul. 13, 2010, application claims the benefit under 35U.S.C. Section 119(e) of U.S. Provisional Application Ser. No.60/815,507, filed on Jun. 21, 2006, by Tadao Hashimoto, Hitoshi Sato,and Shuji Nakamura, entitled “OPTO-ELECTRONIC AND ELECTRONIC DEVICESUSING N-FACE GaN SUBSTRATE PREPARED WITH AMMONOTHERMAL GROWTH,”; and

U.S. Utility patent application Ser. No. 11/977,661, filed on Oct. 25,2007, by Tadao Hashimoto, entitled “METHOD FOR GROWING GROUP III-NITRIDECRYSTALS IN A MIXTURE OF SUPERCRITICAL AMMONIA AND NITROGEN, AND GROUPIII-NITRIDE CRYSTALS GROWN THEREBY,”, now U.S. Pat. No. 7,803,344,issued Sep. 28, 2010, which application claims the benefit under 35U.S.C. Section 119(e) of U.S. Provisional Application Ser. No.60/854,567, filed on Oct. 25, 2006, by Tadao Hashimoto, entitled “METHODFOR GROWING GROUP-III NITRIDE CRYSTALS IN MIXTURE OF SUPERCRITICALAMMONIA AND NITROGEN AND GROUP-III NITRIDE CRYSTALS,”;

which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to gallium nitride bulk crystals and methods formaking the same.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [Ref. x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

The usefulness of gallium nitride (GaN), and its ternary and quaternaryalloys incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), hasbeen well established for fabrication of visible and ultravioletoptoelectronic devices and high-power electronic devices. These devicesare typically grown epitaxially on heterogeneous substrates, such assapphire and silicon carbide, since GaN wafers are very expensive. Theheteroepitaxial growth of group III-nitride causes highly defected oreven cracked films, which deteriorate the performance and reliability ofthese devices.

In order to eliminate the problems arising from the heteroepitaxialgrowth, GaN wafers sliced from bulk GaN crystals must be used. However,it is very difficult to grow a bulk crystal of GaN, since GaN has a highmelting point and high nitrogen vapor pressure at high temperature.

Up to now, a few methods such as high-pressure high-temperaturesynthesis [Ref 1, 2] and a sodium flux method [Ref. 3, 4] have been usedto obtain bulk group III-nitride crystals. However, the crystal shapeobtained by these methods is a thin platelet because these methods arebased on a Ga melt, in which nitrogen has very low solubility and a lowdiffusion coefficient.

A new technique is based on supercritical ammonia, which has highsolubility of source materials, such as polycrystalline GaN or Ga metal,and which has high transport speed of dissolved precursors. Thisammonothermal method [Ref 5-9] has a potential for growing large GaNcrystals. However, existing technology is limited by the crystal size,because the growth rate is not fast enough to obtain large crystals.

SUMMARY OF THE INVENTION

The present invention discloses a gallium nitride (GaN) bulk crystalhaving a polyhedron shape which could not be obtained using existinggrowth methods. The shape of the GaN bulk crystals has an advantage overthe existing platelet-shaped GaN, since GaN wafers of any preferableorientations can be obtained simply by slicing the polyhedron. The GaNbulk crystals have exposed {10-10} m-planes and an exposed (000-1)N-polar c-plane, wherein the surface area of the exposed (000-1) N-polarc-plane is more than 10 mm² and the total surface area of the exposed{10-10} m-planes is larger than half of the surface area of (000-1)N-polar c-plane.

The GaN bulk crystals were grown by the ammonothermal method. Ahigh-pressure vessel made of Ni—Cr based superalloy, having a longestdimension along the vertical direction, is used to contain high-pressureammonia at temperatures above 500° C. The high-pressure vessel isequipped with baffle plates which divide the inner room of thehigh-pressure vessel into two regions along the longitudinal directionof the high-pressure vessel, noted as an upper region and a lowerregion. The Ga-containing source materials, such as polycrystalline GaNor Ga metal, are placed in the upper region and seed crystals, such assingle crystal GaN, are placed in the lower region. To increase thegrowth rate, an alkali-base mineralizer such as KNH₂, NaNH₂, LiNH₂, K,Na, Li, Ca(NH₂)₂, Mg(NH₂)₂, Ba(NH₂)₂, Ca₃N₂, Mg₃N₂, MgCl₂, CaCl₂, MgBr₂,CaBr₂, MgI₂, CaI_(e), Mg, Ca, or similar alkali metal, alkali earthmetal containing substance is added. The high-pressure vessel is filledwith ammonia, sealed, and is heated from the outside by multi-zoneheaters to set a temperature difference between the upper region and thelower region. The polyhedron-shaped GaN crystals were grown by settingthe temperature for the lower region (crystallization region) to be ator above 550° C., the temperature for the upper region (dissolutionregion) to be at or above 500° C., and the temperature differencebetween the upper and lower regions to be at or above 30° C. Thesetemperatures are typically maintained for more than 30 days. Then,typically, the high pressure ammonia may be released at a temperaturehigher than 300° C., the vessel may be unsealed at a temperature higherthan 300° C., and the vessel may be cooled.

Although platelet-shaped crystals can be used as seeds, rod-shaped GaNcrystal seeds are preferable for avoiding multiple grain structure ofthe GaN polyhedron. For example, the crystal may be grown on a seedcrystal, wherein the seed crystal is an a-plane oriented gallium nitridewafer, an m-plane oriented gallium nitride wafer, or a c-plane orientedgallium nitride wafer. The a-plane oriented seed crystal may be obtainedby slicing a GaN boule grown by an ammonothermal method. The crystal maybe grown on a rod-shaped GaN crystal having its longest dimension alonga c-axis. A GaN wafer may be sliced from the grown bulk crystal obtainedusing the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic of a high-pressure vessel according to thepreferred embodiment of the present invention.

FIG. 2 is a flowchart illustrating the method according to the preferredembodiment of the present invention.

FIG. 3( a) and FIG. 3( b) are photographs of polyhedron-shaped GaN bulkcrystals wherein the minimum graduation of the scale shown in FIGS. 3(a) and 3(b) is a half millimeter, and FIG. 3( c) is a schematic top viewof the GaN bulk crystal of FIG. 3( b).

FIG. 4 is a photograph of GaN grown on a hexagonal rod-shaped seed.

FIG. 5( a) is a side view of a GaN boule before slicing, wherein theparallel lines and rectangle represent approximate wire saw position andposition of seed crystal, respectively, and FIGS. 5( b)-(e) arephotographs of c-plane GaN wafers sliced from the GaN boule shown inFIG. 5( a), wherein the minimum graduation of the scale shown in FIGS.5( b)-(e) is a half millimeter.

FIG. 6( a) is a top view of a GaN boule before slicing, and the parallellines and trapezoid represent approximate wire saw position and positionof seed crystal, respectively, and FIGS. 6( b)-(h) are photographs ofm-plane wafers sliced from the GaN boule shown in FIG. 6( a), whereinthe minimum graduation of the scale shown in FIGS. 6( b)-(h) is a halfmillimeter.

FIGS. 7( a)-(c) are schematic cross-sections of examples of seedcrystals used in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Technical Description

The present invention describes a GaN wafer that is obtained by slicinga polyhedron-shaped GaN crystal along any of a number of differentpreferable orientations. The polyhedron-shaped GaN bulk crystal hasexposed {10-10} m-planes and an exposed (000-1) N-polar c-plane, whereinthe surface area of the exposed (000-1) N-polar c-plane is more than 10mm² and the total surface area of the exposed {10-10} m-planes is largerthan half of the surface area of (000-1) N-polar c-plane.

The polyhedron-shaped GaN bulk crystals are grown in supercriticalammonia by using Ga-containing source materials, typicallypolycrystalline GaN or metallic Ga. An autoclave, which has a longdimension along the vertical direction, is used to contain supercriticalammonia at temperature(s) of 500° C. or higher and pressure(s) of 1.5kbar or higher. To hold high-pressure at temperature(s) higher than 300°C., Ni—Cr based superalloy is used as the vessel material.

FIG. 1 is a schematic of a high-pressure vessel 10 according to thepreferred embodiment of the present invention. The high-pressure vessel10 comprises an autoclave body 10 a (which includes an inner room 10 b,an upper region 10 c, a lower region 10 d, and a bottom 10 e), anautoclave lid 20, autoclave screws 30, a gasket 40, one or more baffleplates 50 and an ammonia inlet and outlet port 60. The baffle plates 50(or flow-restricting plates) divide the inner room 10 b of the autoclavebody 10 a into two regions 10 c, 10 d along a longitudinal direction H,which are referred to as an upper region 10 c and a lower region 10 d,and the inner room 10 b has a bottom 10 e. The autoclave body 10 a, theautoclave lid 20, and the autoclave screws 30 may be made of a Ni—Crbased alloy, and have a longest dimension D along a vertical directionV.

Although not shown in FIG. 1, an internal chamber can be used to realizesafe operation and pure crystal growth. Since the total volume of theinner room 10 b required to grow large GaN crystals is very large, thenecessary amount of anhydrous liquid ammonia often exceeds 100 g. Byusing an internal chamber, one can easily liquefy a large quantity ofammonia in the chamber, avoid moisture contamination, and avoid impurityincorporation into the grown crystals.

A similar structure is disclosed in U.S. Utility patent application Ser.No. 11/784,339, filed on Apr. 6, 2007, by Tadao Hashimoto, Makoto Saito,and Shuji Nakamura, entitled “METHOD FOR GROWING LARGE SURFACE AREAGALLIUM NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIA AND LARGE SURFACE AREAGALLIUM NITRIDE CRYSTALS,”, which application claims the benefit under35 U.S.C. Section 119(e) of U.S. Provisional Patent Application Ser. No.60/790,310, filed on Apr. 7, 2006, by Tadao Hashimoto, Makoto Saito, andShuji Nakamura, entitled “A METHOD FOR GROWING LARGE SURFACE AREAGALLIUM NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIA AND LARGE SURFACE AREAGALLIUM NITRIDE CRYSTALS,”, which application is incorporated byreference herein.

FIG. 2 is a flowchart illustrating the method according to the preferredembodiment of the present invention.

Block 70 represents the step of placing Ga-containing materials, GaNsingle crystalline seeds, and mineralizers in the autoclave body 10 a,wherein the Ga-containing materials are typically loaded in the upperregion 10 c of the inner room 10 b and the GaN seed crystals aretypically placed in the lower region 10 d of the inner room 10 b.Mineralizers containing alkali metal or alkali earth metal are alsoadded in either region 10 c or 10 d.

Block 80 represents the step of sealing the high-pressure vessel 10,wherein after loading all solid materials in the autoclave body 10 a,the autoclave lid 20 of the autoclave body 10 a is sealed by tighteningthe autoclave screws 30.

Block 90 represents the step of filling the container (inner room 10 b)with ammonia, wherein ammonia is fed through the ammonia inlet andoutlet port 60 of the autoclave body 10 a.

After the ammonia charge, the high-pressure vessel 10 is isolated byclosing a high-pressure valve (not shown in FIG. 1) connected to theport 60. In this way, all solid materials and ammonia can be loaded intothe high-pressure vessel 10 without any oxygen and moisturecontamination.

Block 100 represents the step of heating the high pressure vessel 10,wherein the autoclave body 10 a is heated with multi-zone heaters to seta temperature difference between the upper region 10 c and the lowerregion 10 d. This way, the source materials are dissolved in thesupercritical ammonia, transported to the seed crystals, and GaN iscrystallized on the seed crystals.

The key feature of the present invention is to set the temperature ofthe lower region 10 d of the inner room 10 b at or above 550° C., setthe temperature of the upper region 10 c of the inner room 10 b at orabove 500° C., while maintaining the temperature difference between thelower 10 d and upper 10 c regions at or above 30° C. By setting thissufficiently large temperature difference, a high growth rate of GaN isachieved. Although growth rate on crystallographic facet planes variesdepending on growth method and conditions, the present invention hasdiscovered that the ammonothermal method in the examples shown belowpreferentially produce polyhedron-shaped GaN having m-plane facets.

Block 110 represents the step of holding the high-pressure vessel 10 athigh temperature, wherein the high-pressure vessel 10 may be held atthese temperatures for 30 days, and more typically, between 60 days and120 days.

Block 120 represents the step of releasing high-pressure ammonia at hightemperature, wherein after the holding step of block 110, the ammonia isreleased via port 60 at high pressure and high temperature.

The high-pressure vessel 10 can then be unsealed at high temperature, asrepresented by Block 130, and subsequently cooled down, as representedby Block 140.

The end results obtained by the method are one or more polyhedron shapedGaN crystals, as represented by Block 150. For example, the end resultmay be a GaN crystal in a polyhedron shape having exposed {10-10}m-planes and an exposed (000-1) N-polar c-plane. A surface area of theexposed (000-1) N-polar c-plane may be more than 10 mm² and a totalsurface area of the exposed {10-10} m-planes may be larger than half ofthe surface area of the (000-1) N-polar c-plane.

Another key feature of the present invention is to use GaN seed crystalswhich have the longest dimension along the c-axis and exposed large aream-planes, a-planes, other nonpolar planes or semipolar planes. The shapeof the seed crystal can be a hexagonal rod, a hexagonal pyramid, m-planeplatelet, a-plane platelets, other nonpolar platelets or semipolarplatelets.

Since conventional c-plane GaN platelets grown by hydride vapor phaseepitaxy (HVPE) have an inherently bent crystallographic lattice, growthon these conventional GaN platelets by the ammonothermal method resultsin a multiple grain structure. By using seed crystals having the longestdimension along the c-axis, a multiple grain structure is avoided. Inaddition, since the impurity and defects incorporation is higher for −cplane than +c plane, growth on a c-plane platelet, as is carried outconventionally, results in uneven distribution of impurities/defectsalong the c-axis (i.e. one side of the crystal has higherimpurity/defects density than the other side). By growing on m-plane,a-plane, other nonpolar planes or semipolar planes, GaN crystals withuniform impurity/defect concentration can be obtained.

The rod-shaped, pyramid shaped, or platelet-shaped GaN can be obtainedfrom a spontaneously nucleated crystal using HVPE or other methods suchas high-pressure synthesis, flux methods, or the ammonothermal method.Since these spontaneously nucleated crystals are grown without stress,the crystal lattice is not bowed. Therefore, the crystals grown by theammonothermal method on these seed crystals are not subject tocrystallographic lattice bowing.

Experimental Results Example 1 (Run Number THV2008) (not Using anInternal Chamber)

In this example, GaN was grown in a high-pressure vessel comprising anautoclave having an inner diameter of 1 inch, inner height of 10 inchesand three baffle plates at the middle height of the chamber. First,4.377 g of NaNH₂ was loaded at the bottom of the autoclave. Then,platelet-shaped c-plane GaN seed crystals (one in a rectangular shapehaving a longest dimension of about 4.5 mm, and the other in atriangular shape having a longest dimension of about 6 mm) were loadedin the lower region of the autoclave and 30 g of polycrystalline GaN,contained in a Ni—Cr mesh basket, was loaded in the upper region of theautoclave. These solid sources were loaded in a glove box in which theoxygen and moisture concentration is controlled to be less than 1 ppm.After the lid of the autoclave was tightened, 45.7 g of anhydrous liquidammonia were condensed in the autoclave. The autoclave was heated by theexternal heater. The lower region was maintained at 575° C. and theupper region was maintained at 510° C. The temperature difference was65° C. and the resulting maximum pressure was 32,270 psi (2.1 kbar). Theautoclave was maintained at high temperature for 82 days and the ammoniawas released after 82 days. As soon as the ammonia pressure wasreleased, the screws of the autoclave lid were loosened, and theautoclave was cooled. At room temperature, the autoclave was opened.

The resulting GaN crystals 160 (grown on the rectangular seed crystal),170 (grown on the triangular seed crystal) having a polyhedron shape areshown in FIGS. 3( a) and 3(b) respectively. FIG. 3( a) shows the crystal160 comprises a hexagonal bottom or end surface 180 a (and may alsocomprise a hexagonal surface 180 b at the opposite end of the crystal160). The surface 180 a is an N-polar (000-1) c-plane and the surface's180 a surface area is approximately 47 mm². The crystal 160 furthercomprises sidewalls 190 a, 190 b, 190 c, 190 d, 190 e and 190 f whichare exposed m-planes and have a total area of approximately 54 mm² (i.e.the sum of the areas of surfaces 190 a, 190 b, 190 c, 190 d, 190 e and190 f is approximately 54 mm²). The total area of exposed m-planes islarger than half of the surface area of N-polar (000-1) surface 180 a.FIG. 3( b) shows the crystal 170 comprising a half-hexagonal bottomsurface 200 a (and may also comprise a half hexagonal top surface 200b). The surface 200 a is an N-polar (000-1) c-plane and has a surfacearea of approximately 42 mm². The crystal 160 further comprisessidewalls 210 a, 210 b, 210 c and 210 d which are exposed m-planes andhave a total area of approximately 70 mm² (i.e. the sum of the areas ofsurfaces 210 a, 210 b, 210 c and 210 d is approximately 70 mm²). Thetotal area of exposed m-planes is larger than half of the surface areaN-polar (000-1) plane 200 a. These polyhedron-shaped GaN crystals 160,170 are ready to be cut for substrates having any preferableorientations.

Example 2 (Run Number C119) (Using an Internal Chamber)

In this example, GaN was grown in a high-pressure vessel comprising anautoclave having an internal chamber. The internal chamber has an innerdiameter of 2 inch, an inner height of 10 inches, and three baffleplates at the middle height of the chamber. First, 9.021 g of NaNH₂ wasloaded at the bottom of the internal chamber. Then, hexagonal rod-shapedGaN seed crystal (with m-plane exposed, approximately 2 mm ofpoint-to-point dimension for the bottom surface and approximately 3 mmin height) was loaded in the lower region of the internal chamber and100.7 g of polycrystalline GaN, contained in Ni—Cr mesh basket, wasloaded in the upper region of the internal chamber. These solid sourceswere loaded in a glove box in which the oxygen and moistureconcentration was controlled to be less than 1 ppm. After the lid of theinternal chamber was tightened, 101.5 g of anhydrous liquid ammonia werecondensed in the internal chamber. Then the internal chamber wastransferred to an autoclave and the lid was sealed. The autoclave washeated by the external heater. The lower region was maintained at 700°C. and the upper region was maintained at 509° C. Since this autoclavehas a thick wall, the temperature difference inside the internal chamberwas about 30° C. at most. The resulting maximum pressure was 27,986 psi(2 kbar). The autoclave was maintained at high temperature for 83 daysand the ammonia was released after 83 days. As soon as the ammoniapressure was released, the screws of the autoclave lid were loosened,and the autoclave was cooled. At room temperature, the autoclave wasopened.

The resulting GaN crystal 220 having a polyhedron shape is shown in FIG.4. As shown in FIG. 4, the bottom N-polar (000-1) c-plane surface 230has a darker color than the m-plane sidewall(s) 240 a, 240 b, 240 c, 240d, 240 e, and 240 f. Since the origin of the color is thought to be dueto impurities and defects, this result demonstrates that growth on anm-plane sidewall 240 a, 240 b, 240 c, 240 d, 240 e, or 240 f involvesless impurity and/or defect incorporation. Also, if this crystal 220 isgrown to a larger size, much less grain structures than crystals growninto conventional c-plane platelets can be obtained.

Example 3 (Wafers Sliced from THV2008)

In this example, the grown GaN boules 250 and 260 shown in FIGS. 5( a)and 6(a) were sliced with a wire saw. The dimension of the grid in FIGS.5( a) and 6(a) is 1 mm. FIG. 5( b), FIG. 5( c), FIG. 5( d), and FIG. 5(e) show c-plane wafers and FIG. 6( b), FIG. 6( c), FIG. 6( d), FIG. 6(e), FIG. 6( f), FIG. 6( g), and FIG. 6( h) show m-plane wafers. Thisexample demonstrates the ease of fabricating wafers with any favorableorientations.

In FIG. 5( a), the parallel lines 270 represent approximate wire sawpositions to cut the slices shown in FIG. 5( b), FIG. 5( c), FIG. 5( d),and FIG. 5( e), wherein the slices of FIG. 5( b), FIG. 5( c), FIG. 5(d), and FIG. 5( e) correspond to the ordering of parallel lines 270 fromtop to bottom. The rectangle 280 in FIG. 5( a) shows the position of theseed crystal for the growth of the boule 250.

In FIG. 6( a), the parallel lines 290 represent approximate wire sawposition to cut the slices of FIG. 6( b), FIG. 6( c), FIG. 6( d), FIG.6( e), FIG. 6( f), FIG. 6( g), and FIG. 6( h), wherein the slices ofFIG. 6( b), FIG. 6( c), FIG. 6( d), FIG. 6( e), FIG. 6( f), FIG. 6( g),and FIG. 6( h) correspond to the ordering of parallel lines 290 from topto bottom. The trapezoid 300 in FIG. 6( a) shows the position (i.e.outline) of the seed crystal for the growth of the boule 260. The singleheaded white arrows in FIGS. 6( b)-(h) represent the lateral boundarybetween the seed crystal and the crystal 260 grown by the ammonothermalmethod.

In addition, analysis of the slice of FIG. 5( b) shows that growth alongthe a-direction is much faster than growth along the m-direction, andthe domain growing along the a-direction is much more transparent thanthe domains growing along the +c or −c directions. Therefore, it isadvantageous to use a-plane seed crystals to grow polyhedron GaN.

Seed Crystals

FIGS. 7( a), 7(b) and 7(c) illustrate various shapes for seed crystals.FIG. 7( a) illustrates a rectangular seed crystal wafer 310 comprisingrectangular-shape 320 surface 330, rectangular shape surface 340,rectangular shape 350 surface 360, and longest dimension 370, as used togrow the crystal 160 of FIG. 3( a). In Example 1, the longest dimension370 is approximately 4.5 mm. The slices shown in FIG. 5( b)-(e) areparallel to the surface 330 of the seed crystal 310, and illustrate therectangular shape 320 and longest dimension 370. The rectangle 250 inFIG. 5( a) shows the perimeter of surface 360. If surface 330 isc-plane, then surface 360 is m-plane and surface 340 is a-plane.

FIG. 7( b) illustrates a seed crystal wafer 380 with triangular-shape390 surface 400, surface 410, and longest dimension 420, as used to growthe crystal 170 of FIG. 3( b). In Example 1, the longest dimension 420is 6 mm. The triangular shape 390, and the longest dimension 420, arealso illustrated in FIG. 6( a). In Example 1, surface 410 is a m-plane,and the trapezoid 300 of FIG. 6( a) outlines a surface parallel to ana-plane, surface 410 (m-plane) and another m-plane (i.e. the plane ofthe paper in FIG. 6( a) is c-plane).

FIG. 7( c) illustrates a hexagonal rod-shaped seed crystal 430 withexposed m-plane 440 a, 440 b, 440 c surfaces, point 450 a-to-point 450 bdimension 460 for the polyhedron shaped (e.g. hexagonal) bottom surface470, and height 480. The crystal 220 in FIG. 4 is grown on a seed 430with dimension 460 of 2 mm and height 480 of 3 mm.

Thus, the GaN crystal 160, 170, 220 may be grown, for example, on a seedcrystal and the seed crystal may be a wafer 310 or 380 or a rod-shapedgallium nitride crystal 430 having its longest dimension 480 along ac-axis. The wafer 310, 380 may be an a-plane oriented GaN wafer(wherein, for example, the largest surface 330, 400 of the wafer 310,380 is an a-plane), an m-plane oriented GaN wafer (wherein, for example,the largest surface 330, 400 of the wafer 310, 380 is an m-plane), ac-plane oriented GaN wafer (wherein, for example, the largest surface330, 400 of the wafer 310, 380 is an c-plane). The a-plane oriented seedcrystal 310, 380 may be obtained by slicing a GaN boule (e.g. 160, 170)grown by an ammonothermal method, for example.

The crystal 160, 170, 220 may grow on all surfaces of a seed crystal310, 380, 430, so as to encapsulate or surround the seed crystal 310,380, 430. For example, the crystal may grow on the a-plane surface ofthe seed crystal, the m-plane surface of the seed crystal, and thec-plane surface of the seed crystal. Typically more crystal is grown onan a-plane surface of the seed crystal than an m-plane surface of theseed crystal, however, because a-plane grows faster than m-plane.

Possible Modifications and Variations

Although polycrystalline GaN was used as a source material in theexamples, the same effect may be obtained using Ga metal, amorphous GaN,or other Ga containing materials as source materials.

Although NaNH₂ was presented in the examples, the same effect can beachieved using an alkali-base mineralizer such as KNH₂, NaNH₂, LiNH₂, K,Na, Li, Ca(NH₂)₂, Mg(NH₂)₂, Ba(NH₂)₂, Ca₃N₂, Mg₃N₂, MgCl₂, CaCl₂, MgBr₂,CaBr₂, MgI₂, CaI_(e), Mg, Ca, or similar alkali metal, or alkali earthmetal containing substance(s).

This method can be used for growing other III-nitrides such as AlN andInN.

Advantages and Improvements Over Existing Practice

The existing methods limit the shape of the GaN crystal to a platelet.The present invention discloses GaN bulk crystals 160 in a polyhedronshape having exposed {10-10} m-planes 190 a-c and an exposed (000-1)N-polar c-plane 180 a, wherein the surface area of the exposed (000-1)N-polar c-plane 180 a is more than 10 mm² and the total surface area ofall the exposed {10-10} m-planes 190 a, 190 b, 190 c, 190 d, 190 e and190 f is larger than half of the surface area of (000-1) N-polar c-plane180 a. This shape of GaN crystals has an advantage over the existingplatelet-shaped GaN since GaN wafers of any preferable orientations canbe obtained simply by slicing the polyhedron. The GaN bulk crystals weregrown by the ammonothermal method with higher temperature andtemperature difference than is used conventionally.

REFERENCES

The following references are incorporated by reference herein.

1. S. Porowski, MRS Internet Journal of Nitride Semiconductor, Res. 4S1,(1999) G1.3.

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3. M. Aoki, H. Yamane, M. Shimada, S. Sarayama, and F. J. DiSalvo, J.Cryst. Growth 242 (2002) p. 70.

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9. U.S. Pat. No. 6,656,615, issued Dec. 2, 2003, to R. Dwilinski, R.Doradzinski, J. Garczynski, L. Sierzputowski and Y. Kanbara, andentitled “Bulk mono crystalline gallium nitride.”

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A method for growing bulk gallium nitride (GaN)crystals, comprising: growing a gallium nitride crystal having apolyhedron shape with exposed {10-10} m-planes and an exposed (000-1)N-polar c-plane, wherein a surface area of the exposed (000-1) N-polarc-plane is more than 10 mm² and a total surface area of all of theexposed {10-10} m-planes is larger than half of the surface area of theexposed (000-1) N-polar c-plane.
 2. The method of claim 1, wherein theexposed (000-1) N-polar c-plane and the exposed {10-10} m-planes aregrowth surfaces of the crystal.
 3. The method of claim 1, wherein thecrystal is grown using an ammonothermal method in supercritical ammonia.4. The method of claim 3, wherein the crystal is grown in ahigh-pressure vessel, and the growing step further comprises heating afirst region of the high-pressure vessel at or above 550° C., andheating a second region of the high-pressure vessel at or above 500° C.,while maintaining a temperature difference between the first region andsecond region at or above 30° C.
 5. The method of claim 3, wherein thecrystal is grown on a seed crystal and the seed crystal is an a-planeoriented gallium nitride wafer.
 6. The method of claim 5, wherein thea-plane oriented seed crystal is obtained by slicing a GaN boule grownby an ammonothermal method.
 7. The method of claim 3, wherein thecrystal is grown on a seed crystal and the seed crystal is an m-planeoriented gallium nitride wafer.
 8. The method of claim 3, wherein thecrystal is grown on a seed crystal and the seed crystal is a c-planeoriented gallium nitride wafer.
 9. The method of claim 3, wherein thecrystal is grown on a rod-shaped gallium nitride crystal having itslongest dimension along a c-axis.